In this review article we discuss the types of non-specific forces that are of importance for the interactions between proteins and surfaces. Some of these forces are well understood theoretically, and their properties investigated in detail. The origin of other types of forces is less well understood, or even controversial, and here we will recapitulate the main experimental findings. Most of our knowledge of interfacial forces stems from the use of various surface force techniques. Two such techniques are described in this article. The typical experimental procedure is outlined and attention is drawn to the most common complications that we have met during our studies. In this article we have chosen to present data that illustrate the presence of some particular type of force, that illustrate a particular complication that occurs when evaluating the experimental findings, or that illustrate the advantage of using electron spectroscopy for chemical analysis (ESCA) in combination with surface force measurements. No system is discussed in detail, and for more detailed information we will refer the reader to the original literature. However, we present an estimate of the amount of small ions co-adsorbed with the protein calculated from a combination of the ESCA and surface force apparatus (SFA) results. This estimate is based on a simple charge balance which requires a knowledge of the adsorbed amount and the diffuse layer charge. The adsorbed amount is obtained from ESCA measurements using a quantification method that utilizes the known chemical composition of the mica surface. The diffuse layer charge is obtained from the magnitude of the double-layer force measured with a surface force technique.